Vapor generator operation



Feb. 23, 1954 H. P. LEWIS 2,669,977

VAPOR GENERATOR OPERATION Filed Aug 21, 1951 v 2 Sheets-Sheet 1 L ATTEMPERATOR v 9 ECONQMIZER Ea T PRIMARY SUPERHEATER SECONDARY SUPERHEATER FIG. I

- INVENT05 HENRY R LEWIS v L, Ar

Feb. 23, 1954 H. LEWIS VAPOR GENERATOR OPERATION 2 Shets-Sheet 2 Filed Aug. 21, 1951 FIG. ,3

INVENTOR' HENRY P. L EWIS I a 3 .h) T TL M R W W F w m 1C n s m T H" N n/ C m T F D E R a E v D O O m m m w m m w bmazfi :55

Patented Feb. 23, 1954 VAPOR GENERATOR OPERATION Henry F. Lewis, Gleveland Heights, Ohio; assignor to Bailey Meter Company; a, corporation of Delaware Application August 21, 1951, S'erialNo. 242,831

2 Claims.

My invention lies in the field of power generation and particularly in the control of steam temprature in connection with present day vapor generators. Practically all central stationcapacity presently being installed, or on order, in the United States, has rated steam conditions above 800p. s. i. g. and 800F1T; the higheroperating temperature being 1050"FT'I atpressures from 1500p. s. i. g. to 2000 p; s. i. g. andrated load from 500,000 to- 1,000,0'00'110. per hr. The problems involved in the generation and close control of the properties of steam are quite different now than was the case at the time of the inventions in this field which are shown in the prior art.

Superheat temperature control is particularly desirable in thegeneration of steam for the production of electrical energy in large central station power plants. In such plantsthe upper limit of superheat temperature is governed by the materials and construction of theturbine served by the steam. In the interest of turbine eflicien'cy the temperature of the steam delivered to the turbineshould be maintained withinclose optimum Iirnitsthroughout awiderangeof capacities.

As feed water temperatures progressively incrcasethere is less and less work for the boiler proper, with the result that its convection heatabsorbing' surface has disappeared to the point where the modern large utility unitconsists ora water-walled furnace, one or more convection superheaters, aneconomizer' and an air heater. Furnace designis now centering around sufiicien-t water cooling surface to ab'sorb the radiant heat to achieve the required relatively low furnaceexi't gas temperatures.

With the superheat'ing" of 'the steam in one or more convection type heat exchange surfaces, the size and cost of suchsurf-aces becomes a material= factor in the total cost of the unit and any' improvement leading to a reduction the size: of superheaters becomes of considerable importance; Usually these surfacesmust be made of expensive high-alloy tubingto satisfactorily handle the temperatures and: pressures encountered.

Itis-thus aprime desi'd'eratum in the design of such a unit; to proportionthe steamgenerating surfaces and the steam' superheatihg surfaces to give the desired' finalsteam temperatureat normal operatingor rated-load. At'peak load, in excess of the ratedload, thefinal steam'temperaturewillbe inexcess of that desired and correspondingly at lower'rating thesteam temperature wiltnot equal that desired; This due to the characteristic curve of" convection type heat exchangers which have a rising function with load (Fig. 2). It is false economy to design the superheater for desired final steam temperatureat peak load, for all leads below that value would produce steam below the desired temperature. On the other hand, the

design of the superheater toproduce the desired final steam temperature at some rating below the expected operating or rated load would require an excessive cost of super-heating surface and an excessive finalsteam temperature throughout the upper ratings witheonsequent' danger tothe turbine or the necessity of extracting some of the surplus heat from the final superheatedsteam.

To reach the desired high superheated steam temperature, but not exceed it. requires exceedingI-ycareful proportioning of the heat absorbing surfacesbothfor'generating steam and for superheating it. But even if the desired superheated steam temperature be just attained initially by very careful designing atexpected or rated load, the superheated steam temperature Will vary during operationby reason of changes in cleanliness of the heat absorbing surfaces. Slag will form and adhere to the heat absorbing surfaces inthe furnace thereby reducing the effectiveness of such surfacesand raising the furnace outlet temperature of the" products of combustion. Furnaceoutlettemperature will also change with percentage of excess air supplied for combustion, with the characteristics of the. fuel burned, and with the rate of'combustion and the corresponding rate of steam generation. All of these things will therefore affect" the temperature of the gases leaving the furnace and supplied: to thesuperheater surfaces" whether the superheati'n elements are located in. the furnace where they absorb some heat by radiation from the burning fuel and products of combustion or Whether they are located-beyond the furnace where they absorbheatby convectio from the productsof combustion only. I

I' preferably consider a unit which is designed to provide the desired final steam temperature zit-expected or ratedload. Throughout an upper range of rating between this rated load and the peak load possibilityof theunit I consider attemperation of the steam to provide a substantially uniform final total steam temperature as close tothe design value as" possible. Attemperation is known and the particular invention disclosed herein is directed to a preferred method and apparat'us for operating such a unit to provide as nearly uniform as possible final total steam tern"- perature throughout a wide upper range of rat'- mgs.

My invention relates" broadly to fluid heat explied to a fuel fired vapor generator having a,

primary and a secondary superheater series con-, nected with provision for spraying water into the flow path of the superheated steam between the two superheaters for tempering the steam and causing the total temperature of the steam leaving the unit to be maintained within desirable limits. It will be evident that my inven tion may be adapted to other arrangements of power producing or utilizing apparatus and that I have merely chosen by Way of example to illustrate and describe one preferred embodiment.

With operation at or above the rating at which the superheating surfaces are designed to produce the desired final total temperature, the characteristic curve of such convection superheating surfaces indicates that the final temperature would be higher than optimum. This is clearly shown in Fig. 2 wherein the shaded portion indicates what might normally be an excess of temperature in the steam over an upper range of ratings. It is not economical to allow the final temperature to reach the indicated values and then to absorb the excess heat by desuperheating following the convection heating surfaces. Such construction and operation subjects the final superheating surfaces to a considerably greater temperature than would otherwise be necessary or desirable. The reduction of the excessively high final temperature by the introduction of vaporizable liquid provides thermal shock and other disagreeable conditions which are to be avoided. It has been found the most desirable construction and operation is to introduce attemperating vaporizable liquid between two sections of superheating surfaces which, with the proper control, will result in an end product very closely approximating the desired final total temperature. The superheating surfaces are thus not subjected to the greatly excessive temperatures, thermal shock is reduced and efficiency and steadiness of control is enhanced. My particular invention resides in a method and control system utilizing certain variables in the operation of the system as control indexes to the end that optimum final total steam temperature is closely approximated through a wide range of upper ratings.

The attemperator may be of the type disclosed and claimed in the patent to Fletcher et al. 2,550,683 wherein a superheated steam conduit has therein a venturi acting as a part of a thermal sleeve to protect the conduit against thermal stresses. Forwardly of the entrance of the venturi is a spray nozzle through which water is atomized in a conical spray which is enveloped by the high velocity superheated steam. The nozzle is thus disposed in a relatively low velocity zone so that there is a low pressure loss due to minimum turbulence created by the nozzle body. The water leaves the nozzle in the form of a spray cone with a hollow vortex, the outer limits of this cone being within the entrance surfaces of the venturi.

A principal object of the invention lies in the 4 continuous control of the rate of water supplied to the attemperator in accordance with selected variables in the operation of the unit to the end that the final temperature of the steam will be maintained within desired limits.

Another object is to control the attemperator in such a manner that the steam produced by the unit will be within desired limits of total temperature at different rates of output.

Another object is to control the rate of supply of water in accordance with load demand as well as to correct for departure of the condition of the produced steam from desired condition.

A further object is to control the attemperator responsive to an indication of demand as well as responsive to an indication of one or more Variable conditions of the steam.

In the drawings:

Fig. 1 is a somewhat diagrammatic sectional elevation of a vapor generating unit having radiant generating surfaces and convection superheating surfaces.

Fig. 2 is a graph of characteristic values in connection with the operation of such a unit.

Fig. 3 is a schematic showing of the control system of my invention.

It will be appreciated that I am illustrating and describing my invention in a preferred mode of operation and combination of apparatus. For example, while I speak of steam generation and superheating the invention is useful in the generation and superheating of other vapors. Furthermore, while I particularly refer to the burning of pulverized coal in suspension it will be understood that the invention is applicable to a burning of other fuels in suspension, such for example as oil or gas and in fact to solid fuels burned on grates.

Fig. 1 shows in somewhat diagrammatic sectional elevation a typical vapor generator of the size and type herein contemplated and in connection with which I will explain my invention. The generator is of the radiant type having a furnace I which is fully fluid cooled with walls 2 of vertically closely spaced plain tubes constituting the vapor generating portion of the unit. Pulverized fuel and air are supplied for combustion through a plurality of burners 3 firing vertically downward from the uppermost portion of the primary furnace I. Products of combustion pass from the furnace I in the direction of the arrow through the tube screen 4 over a secondary superheater surface 5 and then through a primary superheater 6 followed by a tubular economizer section 1. Total fuel and air for combustion may be under the control of stem pressure or some other demand index as is common practice. Furthermore, induced draft and forced draft may be controlled in normal fashion, the same forming no part of the present invention which is directed particularly to the control of the final temperature of the superheated steam leaving the unit.

It will be observed that steam from the separation drum 8 enters the primary superheater 6 at a location T1 and leaves at a location T2 to pass through an attemperator 9 preferably vertically located external of the unit casing. Steam leaving the attemperator 9 enters the secondary superheater 5 at a location T3 and leaves at a location T4 to the turbine or other point of usage. Hereinafter I will refer to the designation T1, T2, T3 and T4 as representing temperature values of the steam at the respective location in its serial path of flow through the primary superheater,

the attemperator and the secondary superheater, with the understanding that temperature T4. is the final total temperature ofv the'steam leavin the unit and which temperature is to be main tained as nearly as possible to an optimum value which in Fig. .2 I have indicated as 1000: FT'I.

In Fig. 2 I indicate the normal characteristic curve of convection superheating surface as a rising function with the optimum value of 1000 F'I'T crossing the unit rated load at X. In other words, the unit load X is the normal expected operating rating to which the generating and superheating surfaces are proportioned to attain at that rating a final steam temperature of 1000 F. The characteristic curve indicates that at ratings below X load the final temperature will tend to fall off, while at ratings above X load the temperature would tend to be greater than desired. It is through the upper range of rating that I employ attemperation to maintain .or attain as nearly a uniform final total temperature of 1000 F. as is possible. In general the shaded area of Fig. 2 is in effect removed or nullified through attemperation by way of the present invention.

It will be noted that on this particular unit the steam flow is counterfiow to the gas flow and the flue gas passes over the secondary superheater before contacting the primary superheater. Changes in steam temperature caused by changes in gas flow, excess air, ratings, etc. have been found to occur simultaneously at the primary superheater outlet (gas location and at the secondary superheater outlet (gas location I i) Any disturbance is noted on final steam temperature T4 as soon as it i detected at the primary superheater outlet T2. Thus a variat-ion in temperature T2 becomes a valuable index in controlling the final steam temperature T4.

I variables of operation in the control of the rate of supply of water to the attemperator. These are desirably the temperatures T2, T3 and T4. T2 is the temperature of the steam leaving the primary superheater indicative of the heat level at the entrance to the attemperator. T3 is the temperature of the steam leaving the attemperator, so that a knowledge of T2 and T3 indicates the effectiveness of the attemperation. T4 is the final check-back temperature of the steam 1eav- .ing the unit where the temperature is desirably to be maintained at optimum value and varies with T3, the inlet temperature to the secondary superheater, as well as with the heat absorption of the secondary superheater. I have found that these three temperatures cooperating in a method andcontrolsystern are the most desirable guides to be utilized in the control of the attemperator of the present unit.

The temperature T3 leaving the attemperator and entering the secondary superheater is very responsive to any changes in water flow to the attemperator. This temperature must 'vary inversely with boiler rating in order to maintain a constant final temperature at the superheater outlet T4 by -counteracting the rising characteristic of the secondary superheater. Thisis clearly shown in Fig. 2 wherein it is indicated that the water flow rate to the attemperator increases with rating and the effect of this increased water flow rate is clearly shown by the increasing divergence between T2 and T3 as rating increases. In other words, T3 is lowered below the expected temperature at beginning of water supply so as to overcome the rising characteristic of the secn general I preferably use three indexes or a ondar-y superheater. v Athigh ratings 1 require the maximum amount of' attempcration hence a lower attemperator outlet temperature, where,- as at lower ratings I require less water, until a point is reached where no attemperation is re;- quired and steam temperature leaving the at.- temperator is the same as the temperatureentering the attemperator.

In general the primary impulse from T: is so adjusted as to callfor a flow of attemperating water increasing with rating. T3 is relatively insensitive and used as a. readiusting factor against the primary control of T2. T4 represents a final desired steam temperature and provides a check-back or readjustment to take care of irregularities in the heat transfer effect of the secondary superheater. While the primary control from is a rough control and the readjustment from T3 is an approximatecontrol to indicate whether the primary control has been satisfactory, it is seen that the final checkback control from Te is a vernier adjustment. The steam temperature leaving the attemperator and entering the secondary superheater is responsive to any changes in water flow to the attemperator. This temperature must vary with boiler rating in order to maintain a constant final temperature at the superheater outlet T4.

In Fig. 3 1 schematically indicate the temperature controllers T2, T3 and T4 which in known manner are receptive to the active element be it thermocouple, resistance element orthe like. of the controllers T2, Ts-and T4 is arranged to position the movable element of -a pilot valve which may be of the general type disclosed and claimed in the patent to Johnson 2,054,464. Pilot valve it, under the control of T4, is arranged to continuously provide in the pipe l6 a fluid loading pressure bearing a desired relation to the value T4. In similar manner the pilot. 11, under the control of controller T3, p s p p 18 a fluid loading pressure continuously representa tive of the value T3. Likewise the pilot t9 provides in the pipe 20 a fluid' load-ing pressure continuously representative of the value T2.

At M I indicate an accelerating relay which may be of the type disclosed and claimed the patent to Fitch 2,441,405. The output of the relay 2i available in the pipe-2'21, isintroduced to the A chamber of a standardizing relay 23 which mayehe of the type disclosed and claimed in the patent. to Gorrie Re. 21,804. The output of the relay '23, available in a pipe "24, is subjectedupon the A chamber of a totalizing relay 2 5 to-the B chamber of which is. connected the pipe Iii and :to the :C chamber of which -i connected the pipe "2 .0. Thus the totalizing relay 25 is receptive of fluid *loading pressures bearing definite relation 'to the-values oi T4, T3 and'iiz respectively. result, available in an outlet pipe 26, passes through ;a hand-automatic. selector -vralv e' 2 .1 to be efiectiveupon the. diaphragm :2Baof acontrol valve .29 located :in the water supply pipe 30:lea.d ing to the :attemperator Thus a-positioning of the valve :20 controls the. rate of supply of --attemperating water to the attemperaltor- 19.

It, will he noted that the fluidiloading pressure "in the :pipe 116, as. W611 as :inthe pipe 20, increases with an increase in temperature T4 or temperatime To. l he'loading pressure in the pipe &8 decreases with an increase in temperature T3. Thus if all three of the temperatures T2, T3 and T4 increase, the effect upon the relay 25 is in the same direction by each of the three loading pressures because the pressures efiective in the chambers A and C are increasing in the same direction and thepressure in the chamber B, opposing chambers A and C, is decreasing. Thus an increase in temperature T2, T3 or T4 will call basically for an increasein rate of water supply to the attemperator 9. This is in agreement with the curves of Fig. 2.

The various elements of the control system are provided with sensitivity and other adjustments so that the efiect of the difierent indexes may be proportioned the one to the other as desired and also the characteristics of the different operating variables may be taken into account. For example, I consider that the temperature T2 is a. primary control and thus I give greater effect to the pressure established in the pipe 29, upon the relay 25, than I do to that which is effective upon the relay through the pipes H3 or 24.

In operation it will be appreciated that, as rating increases toward the right of Fig. 2 past the rating X for which the superheating surfaces are designed, the general intent is for water fiow rate to the attemperator to increase along the characteristic curve at the bottom of Fig. 2. The addition of this attemperating water between the primary and secondary superheaters is to eifect an elimination of the shaded area of the uppermost characteristic curve which otherwise would be a curve characteristic of temperature T4 at the outlet of the secondary superheater.

In plotting T2 the characteristic is relatively the same shape and parallel to the curve T4 and I have already pointed out that temperature T3 must fall ofi faster than temperature T2 increases so as to anticipate the characteristic rise in tem perature with rating of T4 and thus overcompensate the steam by way of the attemperator to take care of the excess heat which would otherwise be put into the steam in the secondary superheater.

With temperature T2 used as a primary or dominant controlling factor, efiective upon relay 25, it will be seen that as T2 varies with rating the valve 29 must be opened along some such characteristic curve as is shown in Fig. 2. Inasmuch as this is the primary and coarse corrective it will be appreciated that it is desirably most eifective upon relay 25 as compared to the effect of the pressures from pipes I8 and 24. A secondary or servient readjusting effect is available through the pipe I 8 under the control of temperature T3 to show the result of adding the attemperating water to the attemperator. Finally, the vernier adjustment by way of the loading pressure in pipe 24, under the control of servient temperature T4, is added into the system so that the final temperature T4 will most nearly follow the desired final temperature as shown on Fig. 2.

It will be appreciated that I have chosen to illustrate and describe certain preferred embodiments of my invention but it will be understood that the invention may be carried out through other apparatus and means. For example, various types of temperature measuring and controlling instrumentalities may be used and it is not necessary to use fluid loading pressure but the control efiects may be accomplished by other means. It is a principal object of my invention,

however, to interrelate the temperature T2, T3 and T4 in the control of the rate of supply of attemperating water to an attemperator located between a primary and secondary superheaters in a unit of the general type being described.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination including a first convection vapor superheater, an attemperator of the direct contact heat exchange type connected to receive vapor discharged from the first superheater, a second convection superheater connected to receive vapor discharged from the attemperator, said superheaters being subject to the flow of heating gases, means supplying vaporizable liquid to the attemperator for regulating the temperature of the vapor passing therethrough, a dominant temperature controller means responsive to an increase and decrease in the temperature of the vapor leaving the first superheater to increase and decrease respectively the supply of vaporizable liquid, a servient tem perature controller means responsive to an increase or decrease in the temperature of the vapor leaving the attemperator to increase and decrease respectively the supply of vaporizable liquid, and a servient temperature controller means responsive to an increase and decrease in the temperature of the vapor leaving the second superheater to increase and decrease respectively the supply of vaporizable liquid.

2. The combination including a first convection vapor super-heater, an attemperator of the direct contact heat exchange type connected to receive vapor discharged from the first superheater, a second convection superheater connected to receive vapor discharged from the attemperator, said superheaters being subject to the fiow of heating gases, means supplying vaporizable liquid to the attemperator for regulating the temperature of the vapor passing therethrough, a primary controller means responsive to an increase and decrease in the temperature at the outlet of the first superheater to increase and decrease respectively the supply of vaporizable liquid, a modifying controller means responsive to an increase and decrease in the temperature of the vapor at the outlet of the attemperator to increase and decrease respectively the supply of vaporizable liquid, and a fine adjustment controller means responsive to an increase and decrease in the temperature of the vapor at the outlet of the second superheater to increase and decrease respectively the supply of vaporizable liquid jointly controlled by the primary and modifying controller means.

HENRY P. LEWIS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,421,761 Rowand et al June 10, 1947 2,526,898 Powell et al Oct. 24, 1950 2,550,683 Fletcher et al May 1, 1951 OTHER REFERENCES Page 23 of Bulletin G 67, published by Babcock 8: Wilcox Co., Liberty Street, New York 6, New York. 

